A CMOS image sensor is an imaging device built with CMOS technology for capturing and processing light signals. Results produced by the CMOS image sensor can be displayed. A type of CMOS image sensor, called a CMOS Active Pixel Sensor (APS), has been shown to be particularly suited for handheld imaging applications.
CMOS imagers comprise an array of pixel circuits, each of which determines a value for a corresponding image pixel of an image focused on the array. Each pixel circuit includes a photosensitive element or photosensor (e.g., a photodiode or a photogate) for sensing/detecting photons, and may include active transistors (e.g., an amplifier) for reading out and amplifying signals representing a “captured” image.
A CMOS imager may also be used to capture color images. In this case, the array of pixel circuits may employ a color filter array (CFA) to separate red, green, and blue information from a received color image. For example, each pixel circuit may be covered with a red (R), a green (G), or a blue (B) filter, according to a specific pattern, e.g., the “Bayer” CFA pattern. Additional color patterns, including but are not limited to Cyan Magenta Yellow (CMY), Cyan Magenta Yellow Black (CMYK), or Red Green Blue Indigo (RGBI), can be used in imager devices.
As a result of the filtering, each pixel of the color image captured by a CMOS imager having a CFA corresponds to only one color. FIG. 1 illustrates the Bayer scheme for pixel array 20. According to the illustrated Bayer pattern, even rows, Row0 and Row2, for example, contain alternating green 22 (designated as Gr) and red 24 (designated as R) pixels. Odd rows, e.g., Row1 contain alternating blue 26 (designated as B) and green 28 (designated as Gb) pixels.
Typically, while a given red pixel circuit has data that may be used to form an R channel, i.e., the R component of an image, it does not have the kinds of data fundamental to forming a B or G channel. The analogous statement is true for each pixel circuit of another color channel. To determine the R value for a B pixel, and analogously for other combinations, the “missing” R value is estimated by a technique called interpolation whereby R values from selected R pixels near the B pixel are employed in order to estimate the R value that would have been formed by the B pixel, had it been an R pixel. In this way, R, G, and B values may be obtained for every pixel.
Many components of an integrated circuit imaging device, due to imperfections or practical limitations or variations in their designs and layout, etc., may interact to contribute to spatial variation in signal values, depending upon a pixel's location, i.e., sensitivity variation over the array. Further, it is known that a given lens may contribute to the pixel circuits' having varying degrees of sensitivity depending upon their geometric locations in the array. In addition, variations in the shape and orientation of and other elements of the pixel circuits may also contribute to non-uniformity of pixel sensitivity across the imager. This phenomenon can adversely affect the images produced by the imager.
Often times these variations can be measured and partially corrected as they mostly depend on the lens design used and generally do not vary from part to part. Such correction can be done in post-processing of already-acquired image data or during image acquisition (i.e., as the image is read out from the imager).
U.S. patent application Ser. No. 10/915,454 (published as U.S. Patent Application Pub. no. 2006/0033005) proposes a correction scheme for non-uniform sensitivity in the imager array. There is a need and desire in the art for additional image correction methods.